A potentiometer is an electronic component that is used to control the resistance value in a circuit. The operation of a potentiometer is quite similar to that of a variable resistor. Potentiometers are generally used in applications where the value of resistance is required to be varied frequently. It eliminates the need to replace the resistors again and again by providing a range of resistance that can be adjusted manually or automatically as per the requirement. A potentiometer acts as an adjustable voltage divider that allows the user to control the flow of electric current through the circuit. Potentiometer falls under the category of passive electronic devices. A potentiometer is simply known as a pot or preset. Potentiometers are typically used in volume controls of an audio system, to measure the internal resistance of a battery, in brightness and contrast control knobs of television, to measure speed and angle of rotation of servo motors, etc.
Structure of a Potentiometer
A potentiometer is a three-terminal device. Two contacts of a potentiometer are fixed in nature; whereas, one of the contacts is the sliding or rotating contact. The sliding or the rotating contact is the output terminal of the device. The internal structure of a potentiometer consists of a metallic strip on which a manganin or constantan wire is wound. The ends of the wire are connected to the fixed terminals of the potentiometer. The variable terminal of the potentiometer is in contact with the wire wound across the metallic strip. Other types of potentiometers consist of a wooden board on which the manganin or constantan wire is stretched and fixed in form of strips. The wire has a constant cross-section area throughout the length. The strips of the wire are parallel to each other and the length of all segments of the wire are equal. The ends of the wire are attached to the copper strips and are fixed to the wooden board with the help of binding screws.
Working Principle of a Potentiometer
A potentiometer allows the user to change the value of resistance by varying the position of the sliding contact along the length of constant resistance. Here, the input voltage is applied across the fixed terminals of the resistor, while the output voltage is obtained between one of the fixed contacts and the sliding contact. To demonstrate the working principle of a potentiometer, consider a closed circuit that contains a battery, a key, and a potentiometer wire. Here, the battery is connected to the ends of a potentiometer wire through a key. The circuit formed as a result is known as the primary circuit. A steady current flows through the potentiometer wire. An emf source is further connected to the positive terminal of the battery and potentiometer wire in series with a galvanometer, a high resistance, and a jockey. The circuit formed as a result of connecting a galvanometer, a high resistance, and a jockey is known as the secondary circuit. If the potential difference between the positive end of the potentiometer wire and jockey is equal to the voltage of the emf source, then no current flows through the galvanometer, hence no deflection in the galvanometer needle is observed. The length of the potentiometer wire at which zero deflection in galvanometer is seen is known as the balancing length. The potential difference across the balancing length can be evaluated by computing the product of current ‘I’ flowing through the primary circuit, the resistance per unit length of the potentiometer wire ‘R’, and balancing length ‘L’. The magnitude of current flowing through the circuit and the value of resistance per unit length of the potentiometer are constant; therefore, the emf of the cell is directly proportional to the balancing length of the potentiometer. This means that increasing the balancing length would lead to a proportional rise in the potential difference and vice versa.